The present invention relates to a spinning or rotating load cell that measures and transmits linear forces along and moments about three orthogonal axes. More particularly, a system and method for compensating for modulation errors that occur on the output signals from the load cell.
One example of a rotating load cell is a wheel force transducer, which is useful in measuring the loads imparted on a vehicle through the tire and rim assembly. U.S. Pat. No. 5,969,268 discloses one type of wheel force transducer or load cell that has enjoyed considerable success. Generally, the load cell includes a rigid central hub and a rigid annular ring that is concentric with the hub. A plurality of radial tubes joins the central hub to the annular ring. In one embodiment, four radial tubes are used and are spaced at equal angular intervals about a central axis of the hub. Strain sensors are mounted to the tubes and sense strain therein. The strain sensors are connected together so as to provide output signals useful in measuring the forces or moments with respect to three orthogonal axes. However, it is well known to those skilled in the art that other forms of multiaxis transducer such as modular component transducer assemblies, piezoelectric sensing element assemblies, or any other multi-axis load sensing device can provide similar data.
The load cell is particularly well suited for measuring the force and moment components of a rolling wheel. The load cell replaces a center portion of a tire rim, wherein the annular ring is attached to or formed integral with the tire rim, while the central hub is secured to a vehicle spindle. The load cell measures the forces and/or moments as the tire is spinning. As appreciated by those skilled in the art, the loading on each of the plurality of tubes of the transducer is sinusoid in nature because the load cell is spinning. The spinning loads recorded by the load cell can be converted to xe2x80x9cnon-spinningxe2x80x9d vehicle coordinates based on the geometric relationship of the angular position of the wheel and tire assembly.
In most wheel force transducers or other rotating load cells, the signals from the sensing elements related to a rotating coordinate system will have a modulation error. Since outputs corresponding to a xe2x80x9cnon-spinningxe2x80x9d coordinate system are based on outputs related to the spinning coordinate system, the outputs corresponding to a non-spinning coordinate system will also have a modulation error. Typically, this error will occur as xe2x80x9cNxe2x80x9d times per revolution. For example, a load cell of the type described above can exhibit a vertical load (representing the weight of the vehicle) of 1,000 lbs. when driven on a straight and smooth surface. However, due to the modulation error, this load can vary +/xe2x88x9240 lbs. with a frequency four times the rotation of the load cell.
As appreciated by those skilled in the art, modulation error is undesirable, as it introduces error into the acquired data. This error not only makes the data inaccurate by this amount, but it can also be significantly detrimental in durability and fatigue testing because of its cyclical nature. Commonly, the output signals from the load cells are used to generate command files for simulation machines that simulate loading in the laboratory. When modulation errors exist in the command files for durability or other testing, the modulation error introduces a cyclical load command that does not really exist in the original test data.
There thus is an ongoing need to ensure accurate data acquisition in order to measure and predict actual force and moment loading on a spinning load cell.
A process for removing a modulation sinusoidal error from signals of a rotating load cell measuring forces and/or moments with respect to a non-rotating orthogonal coordinate system includes mounting the load cell to the rotating object and obtaining a first set of signals from the load cell, wherein at least one signal of the first set of signals is indicative of a load as the object rotates. A characterization of a modulation error is obtained from the first set of signals. The modulation error is a periodic signal having a frequency greater than a frequency of revolution of the load cell. A second set of signals is obtained from the load cell pursuant to object loading. A modulation error in the second set of signals is calculated as a function of the characterization of the modulation error from the first set. The calculated modulation error in the second set of signals is subtracted from the second set of signals.
Compensation can be provided during loading of the object through analog and/or digital electronics. Likewise, compensation can be provided by computation routines (software routines) during data acquisition, or post-processing of the data. Modules (analog/digital/software) can implement the foregoing process and provide compensation. This process is particularly well suited for removing a majority of the modulation error that exists in a wheel force transducer system that measures spindle loads on a vehicle.